There is growing evidence for a role of the IL-12 p40-bearing monokines in the pathogenesis of autoimmune demyelinating diseases, including multiple sclerosis (MS). The IL-12p40 family consists of IL-12 p70 and the recently discovered IL-23. Each is a disulfide linked heterodimer composed of a common IL-12p40 chain and a unique subunit (p35 for IL-12p70 and p l 9 for IL-23). These two cytokines are produced by activated myeloid ceils and share similar biological functions including stimulation of T cell proliferation, IFNgamma, production and Th1 lineage commitment, all of which have been implicated in the pathogenesis of MS. However, it is becoming apparent that the monokines have potentially important differences as well. For example, recent data suggests that IL-12 p70 and IL-23 exert distinct effects on naive and memory T cell subsets. They might also have different patterns of expression in a disease state such as MS. While several studies have demonstrated elevated IL-12p40 levels in circulating monocytes, cerebrospinal fluid and brain autopsy specimens from patients with MS, none have delineated the relative contributions of IL-12 p70 and IL-23. Since each monokine expresses a unique subunit other than p40 that could potentially be targeted therapeutically, detailed information concerning their respective roles is warranted. In addition, IL-12 p70 and IL-23 (or the unique p19 chain of IL-23) might be more exacting markers of disease activity than the common IL-12 p40 chain. In Aim 1 of this proposal we will test our hypothesis that IL-12 p70 and IL-23 are elevated in peripheral monocytes and that their respective receptors are elevated in circulating T cells from patients with secondary progressive MS. Furthermore we will perform longitudinal studies to assess whether monokine/monokine receptor levels rise in association with disease activity. In Aim 2 we will test our hypothesis that myelin-reactive T cell clones derived from patients with MS are selectively predisposed to upregulate IL-12 receptor (IL-12R) and/or IL-23 receptor (IL-23R) upon antigen challenge. In Aim 3 we will investigate the role of the IL-12 p40 bearing monokines in regulating the expression of IL-12R and lL-23R on myelin-reactive T cell clones. Finally, in Aim 4 we will test our hypothesis that suppression of myelin-reactive T cells by CD4+CD25 + regulatory T cells is enhanced by anti-IL-12 and abrogated by IL-12 stimulation.